Author of the code: James J.P. Stewart Homepage: The official website of MOPAC® is maintained by Stewart Computational Chemistry. http://openmopac.net/ Source: The official sources (up to MOPAC 7.1) are available in the official website. http://openmopac.net/ More recent versions (MOPAC2007, MOPAC2009, MOPAC2012, MOPAC2016) are not open-source. There is plenty of other sites which distribute some flavour of MOPAC - CCL archives, funet archives, SourceForge, GitHub etc. http://www.ccl.net/c